The Effect of Bends on the Long-Range Microwave Inspection of Thermally Insulated Pipelines for the Detection of Water

The detection of corrosion under insulation is an ongoing challenge in the oil and gas industry. An early warning of areas of pipe at risk of corrosion can be obtained by screening along the length of the pipeline to inspect the insulation layer for the presence of water, as water is a necessary precursor to corrosion. In a recent paper [Jones et al. in J. Nondestruct. Eval. 2011] we have shown that long-range detection of water volumes can be achieved with microwave signals, using the structure of the clad and insulated pipeline as a coaxial waveguide, with water volumes presenting an impedance contrast and producing reflections of the incident microwave signal. To achieve long-range inspection of complex pipe networks it is essential that the selected guided wave signal can transmit through structural features such as bends and supports whilst retaining a sufficient signal-to-noise ratio. This paper therefore presents a study on the effects of bends on the propagation of the fundamental Transverse Electromagnetic (TEM) mode. First, numerical simulations are used to study the amplitude of the TEM mode transmitted through a bend, as a function of bend radius and angle. It is found that for typical industrial bends with a bend angle of 90∘ and bend radii between three and five times the pipe diameter, the transmission coefficient varies between 96% and 93%, respectively. As the bend angle varies from 0 to 180∘ the transmission coefficient oscillates with maxima around 0, 90 and 180∘ and minima at around 45 and 130∘. Moreover, the amplitude of the oscillation decreases as the bend radius increases. Then, experimental results on a 12′′ diameter waveguide with a length of 7 m are used to validate the simulated results and are found to be in excellent agreement.